1. Field of the Invention
The present invention relates to a hard facing apparatus for hard facing different kinds of materials to valve heads of intake valves or exhaust valves for use with internal combustion engines.
2. Description of the Related Art
As is well known in the art, because valve heads of intake valves or exhaust valves for use with internal combustion engines are repeatedly brought into abutment against valve seats at a high speed, a high degree of wear resistance is required for valve heads. Therefore, as shown in FIG. 4, a highly wear-resistant material (e.g., a cobalt-based alloy, including tungsten, chromium and carbon) is hard faced by welding (as indicated by 3 in FIG. 4) to a valve head 2 of a valve 1. After that, the hard faced portions are mechanically machined (e.g., ground) into a desired shape.
Heretofore, such hard facing to the valve head has been manually carried out by a worker using a welding rod of a cobalt-based alloy with the valve fixedly held on a holder.
The conventional hard facing method, however, has a problem in that because the hard facing is manually conducted by workers, the welded face material varies in thickness and position depending on the amounts of skill of the workers, and the resulting quality of the welded hard faces is not stable.
Also, in the conventional hard facing method, the hard facing is manually conducted by workers such that the worker manually picks up a valve before hard facing (hereinafter referred to as a valve blank), holds it on a holder, performs the hard facing, and takes out the valve after hard facing (hereinafter referred to as a product valve) for discharging it to a valve stock. This raises another problem in that the productivity per unit time is extremely low.
To solve the above problems, it is conceivable to employ a method of grasping a valve blank by a chuck provided at the distal end of a swing arm on a robot hand, swinging the swing arm so as to supply the valve blank grasped by the chuck to a holder, and performing hard facing by a welding torch disposed in opposite relation to a valve head of the valve blank while the valve blank is rotated about its axis. With this method, however, since the valve blank grasped by the chuck is supplied to the holder by swinging the swing arm, the swing arm is moved along an arcuate path and this property of swinging movement makes it difficult to accurately align the axis of the valve blank with an axis of rotation of the holder. If the axis of rotation of the holder and the axis of the valve blank are significantly offset from each other, the distance between the valve head and the welding torch would be cyclically varied during rotation of the valve blank and the thickness of the hard faced portion on the valve head would not be uniform in the circumferential direction, resulting in a problem of reduced quality of the hard facing.